Ultrasonic occupancy sensing devices are used to detect the presence of moving objects such as a person entering an area of interest, e.g., a room, and when such movement is detected perform a function such as turning on lights in the room. The ultrasonic occupancy sensing device radiates (transmits) high frequency sound waves that are undetectable to the human ear. These sound waves bounce off surfaces, including people. Motion is detected via shifts in frequency that are detected as “Doppler shift” when receiving the reflected sound waves and comparing the frequency thereof to the transmitted sound wave frequency in a frequency mixer and then through a low pass filter.
The relative acoustic strength of the high frequency sound waves is affected by many factors including square footage of desired coverage, partitions, drapes, carpeting, furniture, potential reflection patterns, and the efficiency of the transducer converting electrical energy into acoustic energy. If adjustments in detection sensitivity are required, present technology ultrasonic occupancy sensing devices use either a potentiometer (manual) or algorithms in a microcontroller (automatic) to adjust the amplitude of the received signal. Adjusting the amplitude of the received signal is critical to avoid saturation, e.g., overload, of the receiver circuitry and to accommodate various noise sources such as heavy airflow from a supply register in the ceiling and/or wall.
As multiple ultrasonic occupancy sensing devices are added to adjacent spaces, the total amount of ultrasonic energy increases. The ultrasonic total energy can saturate the controlled areas to the point where manual adjustments become very difficult. The ultrasonic sensors have a synergistic effect that increases with the increase in acoustic energy, making adjustment of the received signal strength more and more difficult. The increase in ultrasonic signal density also makes isolating control of discreet areas very difficult and inconsistent over time as conditions change. Variations in performance are observed as the configuration of an area changes, such as differing number of perimeter doors being closed at different times.
Some ways to mitigate the aforementioned problems have been to use different types of ultrasonic occupancy sensing devices designed for specific size areas with transmitter output amplitudes appropriate for the square footage of the specific area of use. Use of ultrasonic transducers, each driven at a different ultrasonic frequency have been used so that the ultrasonic energies do not accumulate and saturate the total areas being monitored and controlled. Since frequencies and ultrasonic power outputs are not selectable, a plurality of different ultrasonic occupancy sensing devices must be used.